This invention relates to sails for sailing vessels or other pliant lifting structures in which the sail or structure is constructed from a plaint sheet material and has a three dimensional airfoil shape for reaction with the wind.
Conventional sails are presently fabricated from a number of separate flat panels of woven cloth, cloth or film laminates, as well as laminates reinforced with yarns or threads. The flat panels are first cut or otherwise formed into the desired predetermined shape, and adjacent panels are joined together by sewing or broadseaming to provide a sail having a three dimensional or airfoil shape.
In the design of modern sails, one important consideration is the ability to control the strength and stretch characteristics of the fabric in particular directions and in certain locations in the sail body. This is sometimes accomplished by the use of selective, specialized o high strength materials, or by alignment of yarns in a cloth or laminate in the direction of maximum expected stress in the panel and in the total sail.
Rather than using woven cloth to make panels, the Conrad U.S. Pat. No. 4,708,080 and EPO patent 224,729 disclose using flat laminates of film and individual reinforcing threads to make flat panels for a sail, which are later jointed together in a conventional fashion. The resulting structure, however, will still have a number of seams between the adjacent panels, and these seams may contribute to excessive stretch or imperfect load transfer when the sail is placed under load. It is also known to apply reinforcing tapes to the outer surface of a finished sail but this is labor intensive, and the sail fabric between the tapes tends to fatigue earlier than normal.
Proposals have been made to form a sail on the three dimensional surface of a mold. The Gardiner et al. U.S. Pat. No. 2,565,219 discloses the use of overlapping strips of material, which are placed on a mold and heat sealed. This patent does not address the problem of overall stress distribution and compensation therefor in the overall sail.
Wagner patent EP 056,657 discloses the manufacture of a sail from a single piece of material by thermoforming. Reinforcing yarns may be included, but by necessity, these yarns or fibers must also be thermoformable, which would exclude many known types of reinforcing yarns or threads which are relatively non-extensible.